Endless track-driven vehicles are commonly used off-road in difficult terrain and under difficult terrain conditions such as in mud, snow, sand and tundra. For example, tracked vehicles are used in snow country for grooming ski slopes and snowmobile trails, for transporting skiers to back country slopes, for ski resort maintenance work, for snow and mountain rescue and for utility company maintenance work.
Tracked vehicles are generally of two types. Most tracked vehicles are of the two-track type, in which a pair of endless track units, one on each of the opposite sides of the vehicle, support and drive the vehicle. The other type is the four-track, in which four separately driven and independently suspended track units, two in front and two in the rear, support and drive the vehicle.
Four-track vehicles have certain advantages over two-track vehicles under extreme conditions such as on steep slopes and in very rough terrain because of the flexible independent suspension of the four-track units and the constant power available to all four-track units, even while turning. Unlike a two-track vehicle, which relies on the differential speed of the two tracks for turning, the four-track vehicle steers much like a wheeled vehicle. Its endless track units can be physically turned for steering.
Despite the advantages of four-track vehicles over two-track vehicles under extreme terrain conditions, the nature of four-track vehicles is such that there are several inherent problems with the existing designs.
First, there is an inherent problem in transmitting power to the tracks of the four track units. This problem arises because each track must be driven by a single drive wheel having a sprocket with teeth for drivingly engaging the track. The drive wheel receives power from the vehicle engine through a drive train that includes two differentials and four axle assemblies. The nature of the drive train and required vehicle ground clearance dictates that the drive wheel of each track unit be located at the apex of a generally triangular track configuration. All other track-supporting wheels of each track unit are idlers, i.e., undriven guide wheels. These guide wheels are spaced apart along the base of the triangular track unit from end to end thereof.
Ideally, the track that is trained about the drive wheel and guide wheels should be absolutely taut and, therefore, incapable of movement from its pitch line in a direction normal to the direction of the track run, i.e., the path of travel of the track about the drive sprocket and guide wheels. However, the known drive wheels often contribute to the track deviating from this ideal path.
Known drive wheels drivingly engage the track with two of fewer sprocket teeth. For example, the inventions in U.S. Pat. No. 3,787,099 and U.S. Pat. No. 3,857,616, issued Jan. 22, 1974 and Dec. 31, 1974, respectively, disclose using a seven tooth drive wheel; however, as best shown in FIG. 4 of each of those references, only two teeth ever drivingly engage the drive saddles of the track at any given time. With this limited interaction between the drive wheel and track, several problems arise.
For example, the known interaction of the track with the drive wheel on four track vehicles subjects the track to wide variations in applied forces as the track travels around its generally triangular-shaped track run. As a section of the track approaches the drive wheel at the apex of the generally triangular-shaped track run, it is pulled forward by a sprocket tooth, placing that section of the track in tension. Then, as the track travels over the apex, the sprocket tooth pushes that section of the track, rapidly placing it in compression. Because only a small section of the track is in contact with the drive wheel at any given time and the entire driving force is transferred from the engine to the track through this limited contact, the forces acting on the track at that point as well as those one or two sprocket teeth are not only variable, but extremely large.
This combination of large and rapidly varying forces applied to the track as it travels about the apex contributes to premature wear of the track and sprocket teeth and the propulsion related components, increase the amount of slack present in the track, and significantly decrease the fuel efficiency of the vehicle. Slack is also at its greatest at the downstream side of the single drive wheel, where the track is being pushed by the sprocket teeth rather than pulled.
Similarly, because of the limited number of teeth driving the track and the resultant large forces transferred to the track though and the increased slack associated with the premature wear of the track, when each tooth of the drive sprocket drivingly engages a portion of the track, there is a tendency for the sprocket tooth to drive the track downwardly about the sprocket, past the track's pitch line and out of the optimal track run. When this occurs, eventually the frictional engagement between each sprocket tooth and a corresponding portion of the track is overcome, and the engaged portion of the track suddenly releases itself from the engaged sprocket teeth and rebounds or "bounces" back toward and past the normal track run. This so-called "track bounce" sets up heavy track vibrations which are transmitted back through other track unit components to the vehicle chassis and body. Not only can track bounce be noisy, it can also make for an uncomfortable ride for the vehicle operator and any passengers, can limit the speed at which the vehicle may operate effectively, and can cause further premature wearing of parts, particularly in the track and other components of the track unit.
In extreme cases, track bounce can cause the track to skip a tooth of the drive sprocket in a phenomenon known as "track jump." When track jump occurs, there is a loss of power to the track, and this in turn may lead to a loss of vehicle control. The four-track vehicle shown in U.S. Pat. No. 3,787,099 would be especially subject to track bounce.
Second, known methods of positioning the idler wheels relative to the drive wheel create premature wear of the components involved. These methods consist of securing the idler wheels to spindles which are secured to a frame. The frame is then pivotally secured to a journal assembly on the drive wheel axle. The existing journal assemblies have a steel outer journal tube, rigidly secured to the frame, and a steel inner journal tube, operably secured to and supporting the drive sprocket wheel axle. The outer journal tube rotates about the inner journal tube permitting the frame to pivot about the axle. However, the steel rubbing against steel associated with this movement causes these journal tubes to abrade and leads to premature wear of these components, which are costly and difficult to replace.
Third, similarly, known methods of securing the idler wheels to their spindles results in excessive maintenance of the spindles. The known spindle is of a constant diameter along its length and secured to the frame at one end. The idler wheel is secured at the other end of the spindle by a nut with the wheel secured between bearing assemblies containing a seal ring, a seal and a bearing, one on each side of the wheel. In light of the dirt, sand, and snow in which the vehicle operates, this known design typically requires the idler wheels to be greased after approximately every eight hours of operation.
Fourth, in light of the environment in which four-track vehicles typically operate, it is common for ice, sand, mud or other debris to build up around the frame, including on top of the journal assembly. If this build-up goes unchecked, it can grow large and hard enough to disrupt the operation of the track about its path. In extreme cases, this built-up debris can derail the track from the vehicle.
Fifth, known four tracked vehicles typically leave a large and disruptive footprint in their path caused by the metal traction bars of the track becoming imbedded in the terrain and overturning the soil as the vehicle advances. Moreover, the large gaps in the known tracks, needed to permit the teeth of known drive wheels to properly engage the track, result in the track having a decreased surface area in contact with the ground. This smaller surface area combined with the weight of the vehicle permits the track to become deeply imbedded into the terrain during operation of the vehicle, and it thereby increases the damage to the terrain when the track is rapidly removed from the terrain as the vehicle passes by. As a result, even though a four-tracked vehicle may function efficiently in a wide variety of terrains, such a disruptive footprint often precludes these known vehicles from operating in environmentally sensitive and protected wildlife areas.
Following the introduction of four-track vehicles, various attempts were made to solve the first of these problems, namely attempting to reduce the likelihood of the track deviating from its ideal path. More specifically, these attempted solutions were aimed at preventing track bounce and track jump. However, these solutions were limited to implementing various support devices aimed at guiding and supporting the track along its ideal path. For example, shortly after the introduction of the vehicle shown in U.S. Pat. No. 3,787,099, so-called "slides" or "sliders" were installed in each track unit along the track run in the gaps between the drive sprocket and the endmost guide wheels. This alleviated the problem of track bounce and its consequential vibration somewhat, but not completely. The track bounce that still occurred slapped against the slider causing a loud noise and vibration which increased wear of the track.
In the mid-1980's, the sliders were replaced by damper wheels. A damper wheel was positioned on each of the opposite sides of the drive sprocket, in the gaps between the drive sprocket and the endmost guide wheels, with the upper peripheries of the damper wheels close to the track run. While the damper wheels eliminated the damaging track wear caused by the sliders and relieved track bounce somewhat, they did not eliminate track bounce and its attendant problems altogether. The damper wheels could not be positioned sufficiently close to the drive sprocket to eliminate substantial gaps between the damper wheels and sprocket wheel. As a result, the drive sprocket teeth would not disengage the track from the sprocket teeth before the teeth pulled the track downwardly out of the track run. Therefore, track bounce continued to occur when the sprocket teeth finally released the track in the gap on the downstream side of the drive sprocket. This gap could not be closed because of the conflict that would occur between the drive sprocket and the damper wheel axles. As a result, track bounce and its consequences continued to be a problem, even with damper wheels, in four-track vehicles.
More recently, a bridge member spanning between the damper wheels and the drive sprocket wheel has been added so that the uppermost surface of the bridge member and the upper periphery of the damper wheel actually define the track run in the vicinity of the apex of the generally triangular run. This bridge attempts to eliminate track bounce and track jump by forcing the track along the proper pitch line about the apex in spite of any sprocket teeth that may remain drivingly engaged with the track as it passes that point. While the bridge works effectively and is a significant improvement over prior designs, it also requires the addition of significant hardware to the existing design. Moreover, the bridge, like all the other proposed improvements to the four-tracked vehicles, does not address the other four problems with the known designs.